1. Field of the Invention
The invention relates to a method and apparatus for identifying a component of gas mixture.
2. Description of Related Art
Potentially dangerous gas mixtures may be found in many workplace environments, the dangers including the risk of fire or explosion from combustible gases, oxygen enrichment or deficiency and exposure to toxic gases. These dangers are well known and gas detection instruments are available to detect a wide range of gases. These instruments typically contain one or more gas sensors, which give a proportional electrical response dependent upon the concentration of the gas to be detected. If the concentration exceeds allowed concentration limits, then the instrument will provide an alarm to warn nearby personnel, or it may activate other remedial action, such as increasing the ventilation. Gas detection instruments for safety applications are broadly divided into two groups, portable instruments, which are designed to be hand held or worn by the user and provide personal monitoring, and fixed instruments, typically wall mounted, which provide area monitoring.
Combustible gases are often characterized by their lower explosive limit, (LEL), which is the minimum concentration of that particular gas in air, which can support combustion. If the concentration is below the LEL, then the gas will not burn without the continued support of an external ignition source. If the concentration of the gas is greater than the LEL, then once ignited, the combustible gas--air mixture will burn, without the need for an external heat source. Indeed, many combustible gas--air mixtures will explode if ignited at concentrations greater than the LEL. At very high concentrations of the combustible gas, there may be insufficient oxygen to support the combustion, and the combustible gas--air mixture will no longer burn. This upper concentration limit for flammability is known as the upper explosive limit (UEL). The upper and lower explosive limits depend on the gas to be detected, as may be seen from the following data, taken from the CRC Handbook of Chemistry and Physics, 68.sup.th Edition, 1987-1988, CRC Press, Boca Raton, Fla.
______________________________________ Limits of Flammability of Gases in Air Gas LEL (% vol.) UEL (% vol.) ______________________________________ Acetylene HCCH 2.50 80.00 Ammonia NH3 15.5 27.0 Benzene C.sub.6 H.sub.6 1.40 7.10 Hydrogen H.sub.2 4.00 74.24 Methane CH.sub.4 5.00 15.00 Pentane C.sub.5 H.sub.12 1.40 7.80 Xylene C.sub.6 H.sub.4 (CH.sub.3).sub.2 1.00 6.00 ______________________________________
There are three main types of sensors used to detect combustible gases. For general leak detection, metal oxide, especially tin oxide sensors are used. The electrical conductivity of the metal oxide changes when exposed to the combustible gas at high temperatures. These sensors are rarely used for safety monitoring, since they commonly lack the precision necessary for this application.
Infrared sensors typically measure the absorption of the gas at 2940 cm.sup.-1 (.about.3.4 .mu.m), which corresponds to the carbon-hydrogen (C--H) bond stretching frequency. The absorption of the infrared light depends on the number of C--H bonds stretching in the molecule. One of the limitations of infrared detectors is that molecules such as carbon monoxide (CO) and hydrogen (H.sub.2) do not have an absorbance at or near 3.4 .mu.m bond, since they do not have any C--H bonds. Even molecules such as acetylene (HCCH) and benzene (C.sub.6 H.sub.6) which both have C--H bonds often have low sensitivity at 3.4 .mu.m since the triple bond in acetylene and the aromatic ring in benzene shift the absorbance of the C--H stretch to higher frequency. These effects of molecular substitution on the C--H bond vibration frequency are well known, and can be found in standard texts such as D. H. Williams, I. Fleming, "Spectroscopic methods in Organic Chemistry", third Edition, McGraw-Hill book Company, Ltd., London, 1980.
The other major type of sensor for combustible gas is the catalytic bead sensor, which measures heat of combustion. The detector bead of a catalytic bead sensor comprises a small platinum coil encased in a ceramic bead containing precious metal catalysts. The combustible gas enters the sensor and travels to the catalytic bead by natural diffusion. The gas is combusted at the bead surface, aided by the catalysts and the resulting release of heat raises the temperature of the bead. This rise in temperature results in an increase in resistance of the platinum coil, which is normally detected using a Wheatstone bridge. Within the sensor, there is usually a second bead, the reference or compensator bead, which is constructed similarly to the detector bead, without the catalyst. The compensator bead comprises one of the other arms of the Wheatstone bridge, and it is used to cancel out any other non-combustion related responses of the beads, such as changes in ambient humidity or thermal conductivity of the gas. The response of the catalytic bead depends primarily on the heat of combustion of the gas and the rate at which the gas can diffuse to the detector bead.
It is common practice to express the concentration of combustible gases as a percentage of the LEL, and thus 2.5% by volume of methane is 50% LEL. The response of catalytic bead sensors is approximately linear over their useful range (0 to 100% LEL), and setting the empirically determined proportionality constant between the output response and the concentration is called calibration. However, the sensitivity to gas varies with the type of gas. Compared to a relative response to 50% LEL of methane of 1.0, the response to 50% LEL pentane is only about 0.5. A more thorough discussion of catalytic bead sensors may be found in the review by J. G. Firth, "Measurement of Flammable Gases and Vapors" in C. F. Cullis, J. G. Firth (Eds.), "Detection and Measurement of Hazardous Gases", Heinemann, London, 1981.
Many of the commonly encountered toxic gases are detected using amperometric electrochemical gas sensors. A typical electrochemical sensor is usually constructed with two or more electrodes in contact with an electrolyte. The electrode is usually separated from the outside environment by a gas porous membrane, and other diffusion barriers. The gas to be detected enters the sensor and passes through the membrane to the working electrode, where is it either oxidized or reduced; alternatively, the rate of oxidation or reduction of the electrode or another species in electrolyte may be limited by the availability of the toxic gas. The resulting electrical current is proportional to the rate at which the gas is being consumed by the electrode. The output current is therefore usually linearly proportional to the gas concentration, since the response is limited by the rate at which the gas to be detected can diffuse into the sensor.
The nature of the response of the sensor to a toxic gas depends on both the design of the sensor and the nature of the gas. Some gases such as carbon monoxide (CO) and hydrogen (H.sub.2) are oxidized at the electrode, whereas other gases such as chlorine and nitrogen dioxide are usually reduced at the sensor electrode. While the oxidation of carbon monoxide to carbon dioxide (CO.sub.2) is a two-electron process, the oxidation of hydrogen sulfide (H.sub.2 S) to sulfuric acid (H.sub.2 SO.sub.4) is an eight-electron process. Thus, a diffusion limited sensor which responds to both hydrogen sulfide and carbon monoxide will give a stronger response to the hydrogen sulfide, for a given concentration of gas.
The above examples of sensor technology are intended to illustrate that the signal obtained for a combustible or toxic gas depends on both the sensor technology employed, and on the properties of the individual gases. This fact poses a quandary for personnel who risk being exposed to a variety of different gases. If they use a broad band sensor, i.e. a sensor that is sensitive to a wide variety of gas types, then there is the risk that the alarm levels will not be appropriate for any given gas. However, if they instead decide to use sensors selective for a particular gas, then there is the risk that if an unanticipated hazardous gas is present, then it may not be detected at all. In addition, they may have to use instruments which contain many sensors or they may have to carry several instruments, which may be both expensive and cumbersome.
It is now common practice to use a broad band sensor for combustible gases, such as a sensor based on infrared or catalytic bead technology, and to set the alarm levels to match the gas with the least sensitive response. If there is likely to be either methane or pentane in a particular environment, then a catalytic sensor based instrument will usually be calibrated with pentane, since pentane has the lower sensitivity. However, this approach can result in false alarms since a safe concentration of methane will set the instrument into alarm. With a broad band sensor, such as the catalytic bead sensor, it is not currently possible to distinguish whether a response is coming from methane or pentane.
In contrast, toxic gases are usually detected with sensors specific to a particular gas. This difference between the combustible gases and the toxic gases is in part due to the wide variation in risk associated with a toxic gas. For example, carbon dioxide has an OSHA eight-hour permissible exposure limit (PEL) of 5000 ppm, carbon monoxide has a PEL of 50 ppm, sulfur dioxide has a PEL of 5 ppm and chlorine dioxide has a PEL of 0.1 ppm. Another reason for this difference is that it is often easier to fabricate an electrochemical sensor to be selective to a particular toxic gas than to fabricate a catalytic bead sensor to be selective to a particular combustible gas. Some attempts have been made to produce broad band electrochemical gas sensors, but they also suffer from the drawback of deciding where to set the alarm levels. For example, a sensor which gives a response to both sulfur dioxide and carbon monoxide, corresponding to 10 ppm, is five times below the OSHA PEL for carbon monoxide but twice the OSHA PEL for sulfur dioxide, even though both gases are oxidized by two electrons.
Clearly, a method would be desirable that enables a gas detection instrument to identify the gas, and then select the appropriate alarm threshold levels to be used.
Several methods have been developed to identify the components of a potentially hazardous atmosphere. Detector tubes kits are available, in which a series of colorimetric tubes are used, initially identifying the unknown gas by broad chemical classification (e.g. acidic, halogenated or reducing gas), followed by successive iterations until the gas is identified. However, this manual approach is time consuming, cumbersome, and provides the analysis only at a single moment in time ("Drager-Tube Handbook," 8.sup.th Edition, National Draeger Inc, Pittsburgh, Pa.).
In the past, gases were identified by collecting a sample, either on an absorbent, such as activated charcoal, or in a clean gas chamber, followed by laboratory analysis. This method is time consuming and since the analysis has to be performed elsewhere, there is often a considerable delay from the time the sample is taken to the time when the identity of the gas is determined.
Laboratory based methods for gas identification usually involve large, expensive and immobile equipment, such as mass spectrometers, gas chromatographs and infrared spectrometers. Considerable effort has been made to adapt these laboratory instruments for use in the field, and several manufacturers do offer portable gas chromatographs (for example HNU Systems, Newton Mass. 02461 and Viking Instruments Corporation, Chantilly, Va. 20151). Mass spectrometers offer high sensitivity and good selectivity, and despite the difficulties of requiring a high vacuum and other engineering challenges, portable and semi-portable mass spectrometers have been developed, by several companies. For example Foster-Monitor Group, of Cheswick Pa., has a mass spectrometer that can collect a sample and identify the component gases based on their molecular masses. Both the portable mass spectrometers and gas chromatographs are capable of identifying an unknown gas. While these devices offer considerable potential, they remain too expensive for routine safety monitoring, and tend to be used for more specialized applications.
Infra-red spectroscopy can also be used to identify a particular gas. Whereas most infrared combustible gas sensors operate at a single wavelength, and thus have difficulty distinguishing between various hydrocarbons, the full infrared spectra of most organic compounds are unique. Thus, if the full spectrum is obtained with an infrared spectrometer, then the spectrum can be compared against a library of infrared spectra. Due to the complexity of the optics, the cost associated with obtaining a full spectrum and the subsequent data analysis increases the cost of this instrument well beyond that normally used for routine safety monitoring.
Sensor arrays have been developed which are capable of identifying a wide range of gases. These sensor arrays effectively have a large number of sensor elements, each with a different response characteristic. The combined pattern of response from a sensor array can indicate the concentration and identify of gaseous species present. However, sensor arrays have two drawbacks. The first drawback is that the pattern recognition requires very complex mathematical analysis, and thus they require a significant computer analysis to achieve useful results. The second and more important drawback is that the present day sensor arrays do not have the reliability necessary for safety applications. It is likely that both of these problems will be overcome in the future, and sensor arrays are now commercially available, for example from Cyrano Sciences Inc, Pasadena, Calif., and are being used for applications such as food quality and wine classification. The operation of these sensor arrays has been described by M. S. Freund and N. S. Lewis in proceedings of the National Academy of Science (1995), 92, 2652-2656 and by N. S. Lewis in U.S. Pat. No. 5,571,401.
As may be seen from the above discussion, there is a need for an economical method that will identify an unknown gas, so that the appropriate calibration and alarm set point values can be selected. The instrument should be economical enough to be incorporated in personnel and fixed-point safety monitoring equipment. Furthermore, the accuracy and precision of the gas concentration measurement should be as good as the present technology, and preferably the instrument should still use the existing sensor technologies, since they are well tested and have a good service record.
Recently, Zdanevitch et al in U.S. Pat. No. 5,709,792 described a method for identifying a combustible gas using a catalytic bead sensor by varying the potential applied to the bead and using the differences in temperature needed to initiate combustion as a means to identify the gas. This method has many advantageous features, but it suffers from the limitation that the temperature of combustion may vary as the sensor catalyst ages or if the sensor is exposed to poisoning or inhibiting compounds. This deleterious effect will be much more pronounced than is normally observed with a conventional sensor operating under diffusion control at a temperature sufficiently high to oxidize all combustible gases, which reach the detector element of a catalytic bead sensor.